Process for cleaning aluminum

ABSTRACT

AN AQUEOUS CLEANING SOLUTION FOR CLEANING ALUMINUM SURFACES COMPRISING FROM 0.5% TO 15% BY WEIGHT OF CAUSTIC ALKALI, FROM 0.005 TO 0.2% BY WEIGHT OF AN ALKALISOLUBLE SALT OF A POLYCARBOXYLIC ACID HAVING MORE THAN 10 CARBOXYL GROUPS IN THE MOLECULE, AND FROM 0.003% TO 0.1% BY WEIGHT OF ALKALINE EARTH METAL IONS; AS WELL AS THE PROCESS OF CLEANING ALUMINUM SURFACES WHICH COMPRISES TREATING THE SURFACES WITH THE SAID AQUEOUS CLEANING SOLUTION AT A TEMPERATURE OF FROM 50*C. TO 85*C., RINSING AND DRYING THE CLEANED SURFACES.

United States Patent Ofice 3,687,858 Patented Aug. 29, 1972 3,687,858 PROCESS FOR CLEANING ALUMINUM Roland Geisler, Dusseldorf-Holthausen, Georg Jansen, Neuss,'and Hans Gunther Germscheid, Hosel, Germany, assignors to Henkel & Cie G.m.b.H., Dusseldorf-Holthausen, Germany No Drawing. Filed July 23, 1969, Ser. No. 844,184 Claims priority, application Austria, Sept. 2, 1968, A 8,498/68 Int. Cl. Clld 7/06 US. Cl. 252-156 9 Claims ABSTRACT OF THE-DISCLOSURE An aqueous cleaning solution for cleaning aluminum surfaces comprising from 0.5% to 15% by weight of caustic alkali, from 0.005 to 0.2% by weight of an alkalisoluble salt of a polycarboxylic acid having more than 10 carboxyl groups in the molecule, and from 0.003% to 0.1% by weight of alkaline earth metal ions; as well as the process of cleaning aluminum surfaces which comprises treating' the surfaces with the said aqueous cleaning solution at a temperature of from 50 C. to 85 C., rinsing and drying the cleaned surfaces.

THE PRIOR ART Aluminum surfaces are usually cleaned before painting, since the residues left after machining, such as grinding and polishing pastes, drawing assistants and other contaminations, must be removed. Usually, strongly alkaline solutions are used which, in addition to caustic alkalis and alkali metal carbonates, also contain phosphates, silicates, borates, gluconates' and possibly surface-active substances. Even if a certain attack on the metal takes place, the treatment with these agents mostly results in OBJECTS OF THE INVENTION An objectof the present invention is the obtaining of anaqueous cleaning solution for cleaning aluminum surfaces comprising from 0.5 to by weight of caustic alkali,-from 0.005 to 0.2% by weight of analkali-soluble salt of a polycarboxylic acid having more than 10 carboxyl groups in the molecule, and from 0.003% to 0.1% by weight of alkaline earth metal ions.

Another object of the invention is the development of a process for cleaning aluminum surfaces, particularly prior to their being painted, which comprises the steps DESCRIPTION OF THE INVENTION The drawbacks of the prior art have been overcome and the above objects have been achieved by the discovery of an aqueous solution for cleaning aluminum surfaces before painting which comprises from 0.5% to 15 by weight of caustic alkali, from 0.005% to 0.2% by weight of an alkali-soluble salt of a polycarboxylic acid having more than 10 carboxyl groups in the molecule, and from 0.003% to 0.1% by weight of alkaline earth metal ions.

In another aspect, the present invention provides a process for cleaning aluminum surfaces before painting which comprises treating the aluminum surfaces with an aqueous solution comprising from 0.5% to 15 by weight of caustic alkali, from 0.005% to 0.2% by weight of an alkali-soluble salt of a polycarboxylic acid having more than 10 carboxyl groups in the molecule, and from 0.003% to 0.1% by weight of alkaline earth metal ions, at a temperature of from 50 to 85 C., subsequently rinsing and'drying said cleaned surfaces.

The caustic alkalis employed are usually caustic soda or caustic potash solutions. These caustic alkali solutions may be solutions of alkali metal hydroxides such as sodium and potassium hydroxide or commercial caustic alkali solutions containing small amounts of the carbonates.

Suitable alkali-soluble salts of polycarboxylic acids with more than 10 carboxyl groups in the molecule are, especially, salts such as the alkali metal salts of polyacrylic or polymethacrylic acid, hereinafter called polyacrylates or polymethacrylates. It has been found that relatively low molecular weight polyacrylates with a molecular weight from about 3000 also possess a good action. Such acrylates have about 30 carboxyl groups in the molecule. This is further improved, however, if polyacrylates or polymethacrylates with molecular weights up to 100,000 are used. Provided the salts are soluble in alkali solutions, however, products with a molecular weight up to 200,000 can be used. In addition to the preferably utilized polyacrylates or polymethacrylates, alkali-soluble salts of cooftreating aluminum surfaces with an aqueous solution polymers containing more than 10 carboxyl groups which can be used are those such as for example, the copolymers of ethylene and maleic acid, ethylene and acrylic or metha'crylic acid. Further, polyaminopolycarboxylic acids, such as poly-lower-alkyleneimine-N-carboxylic acids, for example, polyethyleneimine-N-acetic acid and polyethyleneimine-N succinic acid, are suitable. Instead of the said salts, which can be used singly or in admixture, the corresponding acids may also be used, since the corresponding salts are formed in the alkaline solutions.

The solutions must also contain alkaline earth metal ions, which may be added in the form of their alkali-solu ble salts. Suitable alkaline earth metal ions are, especially, barium, strontium'and preferably calcium. The addition of the alkaline earth metal ions is suitably synchronized with the amount of alkali-soluble salts of polycarboxylic acids used. That is, if relatively large amounts of the latter are used, it is expedient if a relatively large amount of alkaline earth metal ions is also present in the solutions. If, on the other hand, small amounts of polycarboxylic acid salts are used, only small amounts of alkaline earth metal ions are also needed. The salts of the alkalisoluble polycarboxylic acids are used in amounts from 0.005 to 0.2% by weight, while the addition of alkaline earth metal ions lies in a range from 0.003% to 0.1% by weight, preferably from 0.01% to 0.1% by weight.

In many cases a further addition of hydroxyor aminophosphonic acids with at least 2 phosphorus atoms in the molecule or their soluble salts has a favorable effect on the subsequent adhesion of the paint. Suitable hydroxyphosphonic acids with a least 2 phosphorus atoms in the molecule are, for example, alkali-soluble l-hydroxyalkane-1,1-diphosphonic acids such as l-hydroxyethane- 1,1-diphosphonic acid, l-hydroxypropane 1,1 diphosphonic acid, l-hydroxybutane-l,l-diphosphonic acid, or 1- hydroxyhexane-l,l-diphosphonic acids. Suitable aminophosphonic acids with at least 2 phosphorus atoms in the molecule are l-aminoalkanel,l-diphosphonic acids, preferably l-aminoethane-l,l-diphosphonic acid, or l-aminoaryl-1,l-diphosphonic acids, preferably a-aminobenzyl-a, a-diphosphonic acid.

Also aminomethylenephosphonic acids may be used such as aminotrimethylenephosphonic acid, ethylene diaminetetramethylenephosphonic acid, hexamethylenediaminetetramethylenephosphonic acid or polyethyleneimino-N-methylenephosphonic acid. Instead of the acids their soluble salts, especially the potassium and sodium salts, may also be used.

The phosphonic acids having at least 2 phosphorus atoms in the molecule or their salts are added to the cleaning solutions in a concentration of from 0.005% to 0.2%

i by weight. Furthermore, an addition of polyalcohols, as

for example, sorbitol or mannitol, and /or polyhydroxycarboxylic acids with at least two adjacent OH groups in the molecule has a favorable effect. Gluconic acid or heptagluconic acid is preferably used. Instead of the polyhydroxycarboxylic acids, their alkali soluble salts or their boric acid complexes such as borogluconate and boroheptonate may also be used.

Treatment solutions which contain 0.05% to 10% by weight of caustic alkali, 0.01% to 0.2% by weight of alkali-soluble polyacrylates or polymethacrylates, 0.01% to 0.2% by weight of hydroxyethane-1,1-diphosphonic acids and/or aminotrimethylenephosphonic acids or their salts, 0.01% to 0.2% by weight of gluconic acids and/or heptagluconic acid or their salts or the corresponding boric acid complexes, and 0.01 to 0.1% by weight of calcium ions have proved specially favorable in regard to subsequent paint adhesion, little attack on the aluminum surfaces, good cleaning action and prevention of hardening of the coatings.

Furthermore, the treatment solutions maystill contain the usual constituents of cleaning agents, especially alkali metal compounds such as ortho and polymeric-phosphates, carbonates, borates, silicates, and, if desired, surfaceactive compounds.

The treatment solutions may be used in dipping processes, flooding processes and spray processes. The most favorable treatment temperature lies between 50 and 85 C. At lower temperatures the cleaning efifect is generally not suflicient to attain the desired effect in technically convenient times. At higher temperatures, error easily occur through drying of the surface before the rinsing step.

The strongly alkaline cleaning solutions used may be produced either directly from the components or by dissolving the corresponding compounded solid or liquid cleaning agents. After the treatment with the solution according to the invension the aluminum surfaces are rinsed with water and then dried in a known way. The last rinsing is suitably carried out wtih water containing very little salt or completely demineralized. The aluminum surfaces are satisfactorily cleaned of all adhering contaminations by the process of the invention, and the adhesion of paint is substantially better in the method according to the invention than when the treatment has been only with the previously known aluminum cleaning agents. The process has proved specially advantageous in the cleaning of aluminum spray cans and satisfactory adherence of the paint is guaranteed in such cases even on later deforming of the tube blanks. A further advantage of the method of the invention is the very small removal of surface material caused by the treatment solutions in spite of their high alkalinity. It is frequently less than 10% and in the most favorable case is less than 5% of the corrosion caused when corresponding aluminum cleaning and pickling solutions are used without the additives according to the invention. Surfaces of pure alumi-. num and aluminum wrought alloys may be treated with the cleaning solutions according to the invention. In the case of aluminum casting alloys with a high silicon content, on the other hand, troublesome incrustation may occur in spite of the little attack on the material which make good adhesion of paint impossible. 1

The following examples are illustrative of the invention without being deemed limitative in any manner."

The quantities given in the following examplesarein percent by weight, provided nothing else is indicated.

EXAMPLE 1 Tube blanks molded from pure aluminum (A1 99.5%- DIN 1712) and covered with zinc stearate and mineral oil, were treated in the spray process with a'solution of 1.5% of NaOH, 0.3% of sodium carbonate and 0.03% of sodium gluconate at 70 C. under a spray pressure of 0.75 atm. for 25 seconds, and were then rinsed with circulating water, fresh water and finally with fully-demineralized water. After blowing with compressed air and drying with hotair, the tube blanks were provided by the spray process with, an inner coating of protective paint based on a modified phenolformaldehyde resin and baked at 250 .C. for 8 minutes. I

The strong deformation taking place during the drawing in and hanging of the can material causes peeling off in places of the film of paint in a high percentage of the finished cans. If, on the other hand, the tube blanks were treated with a cleaning solution which contained 1.5% of N aOH, 0.02% of polyacrylate with a specific viscosity of 1 20=0.68 cp. (0.7% solution), 0.02%, of l-hyd roxyethane-1,1-diphosphonic acid, 0.02% of sodium gluconate, 0.02% of Ca('NO the treatment being carried out just as described above, no peeling of the paint was noted after the deformation.

In this latter case the material removed only amounted to approximately 10% of the valueof the first described cleaning solution. p Q

In the same way satisfactory cleaning could be attained in conjunction with an increased adhesion of paint when a mixture consisting of 1% 'of caustic soda, 0.3% of trisodium phosphate, 0.01% of sodium gluconate, 0.01% of polyacrylic acid with a specific viscosity in a 0.7% solution of 1 20=0.12 cp., 0.01% of aminotrirnethy lenephosphonic acid and 0.04% of calcium chloride (corresponding to 0.011% of Ca ions) was used as the alkaline treatment solution. Here also the removal of metal was reduced to about 5% of the value which was obtained with the first-mentioned cleaning solution.

EXAMPLE 2 Metal sheets of quality A1 99.5% (DIN 1712) covered with cylinder oil were'clean'ed with the following alkaline solutions at the given temperature, thoroughly rinsed with water and, after drying in hot air, sprayed with a transparent epoxide resin paint, which was baked on for 6 minutes at 270 C.

The adhering tenacity of the coating was examined by a modified grid sectioning test according to DIN 53,151, whereby, instead of the grid sectioning, 'a scoring of exactly 5.5 mm. was carried out with the Erichsen apparatus (DIN 53,156) and after a quick tearing off of an adhesive strip pasted on the section, the pieces of paint remaining on the surface of the metal are estimated. The quality of the painting was denoted by the grid sectioning characteristic values (DIN 53,151) Gt 0-4:

Gt 0 =no pieces of the coatingbroken off,

Gt 1=about 5% of the pieces,

Gt 2=about 15% of the pieces,

Gt 3: about 35% of the-pieces,

Gt 4: more than.65% of the pieces peeled off.

The cleaning was effected with difierently constituted cleaning solutions and at dilferent temperatures and times of treatment, which are given below in each case. The respective additions to the basic cleaning solutions are given under a-h for 1) and ad for 2) and the grid 5 section characteristic values are given under Gt.

(1) The metal sheets were in each case treated for 2 minutes at 50 C. with cleaning solutions of the following composition:

3 caustic soda plus 0.05% sodium gluconate 3 a) plus 0.03% polyacrylate, 20=0.l2 cp. (0.7% solution) plus 0.02% calcium nitrate 1 (b) plus 0.03% polyacrylato, 1,20=0.22 cp. (0.7% solution) lus 0.02% calcium nitrate 0 (c? plus 0.03% polyacrylate, =0.68 cp. (0.7% solution) plus 0.02% calcium nitrate 0 (d) plus 0.02% polyacrylate, 20=0.l2 cp. (0.7% solution) plus 0.02% l-liydroxyethane-1,1-diphosphonate plus 0.03% calcium nitrate 0-1 (e) plus 0.02% polyacrylate, 1,20=0.12 cp. (0.7% solution) plus 0.02% l-hydroxyhexane-l l-diphosphonate plus 0.03% 20 calcium nitrate 2 (f) plus 0.02% polyacrylate, 20=0.12 cp. (0.7% solution) plus 0.02% amino-trimethylenephosphonic acid plus 0.03% calcium nitrate 0-1 (g) plus 0.02% polyacrylate, 1 20=0.l2 cp. (0.7% solution) plus 0.02% hexamethylenediaminetetramethylenephosphonicacidplus0.03%calciumnitrate 1 (h) plus 0.02% polyacrylate, 20=0.12 cp. (0.7% solution) plus 0.02% pclyethyleneimino-N-polymethylenephosphomc acid plus 0.03% calcium nitrate 1 (2) The metal sheets were treated in each case for seconds at 70 C. with cleaning solutions of the fol- 30 lowing composition:

10% caustic soda plus 2% tn'sodium orthophosphate plus 2% sodiurntetraboratejl l O 3 (a) plus 0.00% polyacrylate, 1,20=0.68 cp. (0.7% solution) plus 0.2% sorbitol plus 0.1% calcium nitrate 7 0 (1)) plus 0.05% polyacrylate, 20=0.68 cp. (0.7% solution) plus .05% a-aminobenzyl-a,a-diphosphomc acid plus 0.2% calcium nitrate plus 0.1% sodium heptaborogluconate 1 (0) plus 0.06% polyethyleneimine-N-succimc acid plus 0.05%

calciumnitrate 2 40 ((1) plus 0.08% polyethyleneimine-N-acetic acid plus 0.05%

calcium nitrate 2 The results show that the metal sheets, which were treated with the cleaning solutions according to the nvention produced in the paint adhesion grid sectiomng test, characteristics of Gt 02, whereas the sheets treated with cleaning solutions of the usual composition had a paint adhesion of only Gt 3.

Besides this, there resulted with the solutions according to the invention a small removal by corrosion of only about 5 to 10% of the amount removed obtained with the ordinary alkaline agents.

The preceding specific embodiments are illustratlve of the practice of the invention. It is to be understood, however, that other expedients known to those skilled 1n the art may be utilized without departing from the spirit of the invention.

We claim:

1. A process for cleaning aluminum surfaces, particularly prior to their being painted, which consists essentially of the steps of treating aluminum surfaces with an aqueous solution consisting essentially of from 0.5% to 15% by weight of caustic alkali, from 0.005% to 0.2% by weight of a caustic alkali-soluble salt of a polycarboxylic acid having more than 10 carboxyl groups in the molecule selected from the group consisting of polyacrylates, polymethacrylates and mixtures thereof, having a molecular weight of from 3,000 to 200,000, polyethyleneimine-N-acetic acid and polyethyleneimine-N-succinic acid, and from 0.003% to 0.1% by weight of alkaline 7 earth metal ions selected from the group consisting of barium, calcium and strontium, in the form of their caustic alkali-soluble salts, at a temperature of from 50 C. to 85 C., subsequently rinsing the cleaned aluminum surfaces and recovering said cleaned aluminum surfaces.

2. The process of claim 1 wherein said aqueous solution has a further content of from 0.005% to 0.2% by weight of a caustic alkali-soluble salt of a phosphonic acid having at least 2 phosphonic acid groups in the molecule selected from the group consisting of hydroxyphosphonic acids, aminophosphonic acids and aminomethyl enephosphonic acids.

3. The process of claim 1 wherein said alkaline earth metal ions are present in an amount of from 0.01% to 0.1% by weight.

4. The process of claim 1 wherein said caustic alkali is selected from the group consisting of caustic soda and caustic potash.

5. The process of claim 1 wherein said alkali-soluble salt of a polycarboxylic acid is selected from the group consisting of polyacrylates, polymethacrylates and mixtures thereof, having a molecular weight of from 3,000 to 200,000.

6. The process of claim 1 wherein said phosphonic acid is a l-hydroxyalkane-1,1-diphosphonic acid.

7. The process of claim 1 wherein said aqueous solution has a further content of from 0.005 to 0.3% by weight of a caustic alkali-soluble hexanehexols, gluconates, heptogluconates, boric acid complexes of said gluconates and heptogluconates, and mixtures thereof.

8. The process of claim 2 wherein said aqueous solution has a further content of from 0.005% to 0.3% by weight of a caustic alkali-soluble hexanehexols, gluconates, heptogluconates, boric acid complexes of said gluconates and heptogluconates, and mixtures thereof.

9. A process for cleaning aluminum surfaces, particularly prior to their being painted which consists essentially of the steps of treating aluminum surfaces with an aqueous cleaning solution for cleaning aluminum surfaces consisting essentially of from 0.5% to 10% by weight of caustic alkali, from 0.01% to 0.2% by weight of a caustic alkali-soluble salt of a polycarboxylic acid having a molecular weight of from 3,000 to 200,000 selected from the group consisting of polyacrylates and polymethacrylates, from 0.01% to 0.2% by weight of a phosphonic acid selected from the group consisting of 1- hydroxyethane-1,1-diphosphonic acid, aminotrimethylenephosphonic acid, and caustic alkali-soluble salts thereof, from 0.01% to 0.2% by weight of a compound selected from the group consisting of gluconic acid, boric acid complexes with gluconic acid, heptagluconic acid, boric acid complexes with heptagluconic acid and caustic alkalisoluble salts thereof, and from 0.01% to 0.1% by weight of calcium ions in the form of a caustic alkali-soluble salt, at a temperature of from 50 C. to C., subsequently rinsing the cleaned aluminum surfaces and recovering said cleaned aluminum surfaces.

References Cited UNITED STATES PATENTS 3,308,067 3/1967 Diehl 252--16l 3,326,807 6/1967 Guest et al 252152 3,535,258 10/ 1970 Sabatelli et al. 252 3,255,118 6/ 1966 Carroll et al. 252137 2,993,861 7/ 1961 McCune 252137 3,368,978 2/ 1968 Irani 252152 3,332,880 7/ 1967 Kessler et al 252161 FOREIGN PATENTS 451,342 7/ 1936 Great Britain 252--156 753,208 2/1967 Canada 252156 26,536 11/1969 Japan 252156 666,119 7/ 1963 Canada.

MAYER WEINBLATT, Primary Examiner U.S. Cl. X.R.

134-42; 252DIG 2, DIG 8, 89, 139, 162, 

